Radio system, radio base station and radio terminal

ABSTRACT

One of a plurality of radio terminals that transmit radio signals to a radio base station under a space multiplexing scheme, including a receiving unit that receives an access right request signal with respect to the radio base station from an other radio terminal, a prevention unit that prevents transmission of radio signals when receiving radio signals directed to an other radio terminal, a cancellation unit that cancels the prevention of transmission when receiving an access right assignment signal during the prevention of transmission and a transmitting unit that transmits a data signal to the radio base station according to the assignment signal, wherein radio signals are transmitted to the radio base station simultaneously with the other radio terminals and under the space multiplexing scheme.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 12/441,571, filed on Mar. 17, 2009, which is a Continuation ofPCT International Application No. PCT/JP2009/051916 filed on Jan. 29,2009, the entire contents of both of which are incorporated herein byreference.

This application claims benefit of priority under 35 USC §119 toJapanese Patent Application No. 2008-87988 filed on Mar. 28, 2008, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a radio system, radio base station andradio terminal.

BACKGROUND ART

In frequency bands of 2.4 GHz and 5 GHz, wireless LAN systems compliantthe IEEE802.11 standard using a frequency band of 20 MHz are becomingwidespread at an accelerating pace. The IEEE802.11 standard(IEEE802.11a/b/g) which has already been put to practical use adopts anaccess scheme of CSMA/CA (carrier sense multiple access with collisionavoidance).

Furthermore, the Task Group n (TGn) is engaged in new standardizationactivities aiming at the achievement of throughput of 100 Mbps or higherand is studying, for example, a MIMO (Multi Input Multi Output)technique that carries out communication using a plurality of antennasand an aggregation technique that aggregates a plurality of packets intoone packet to be transmitted/received in a MAC layer.

The activities intended to speed up communication of wireless LANsystems are believed to continue, and in practice, an IEEE802.11standardization conference as a study group has started to study on thefurther speed enhancement and improvement of the system capacity ofwireless LAN systems.

As one of schemes leading to the improvement of system capacity of awireless LAN system, there is known a Space Division Multiple Access(SDMA) scheme, that is, a scheme whereby data is transmitted/receivedbetween a radio base station and a plurality of radio terminalssimultaneously and at the same frequency (spatially multiplexed) (e.g.,JP-A 2003-52079 (Kokai)).

When a wireless LAN system is constructed by adopting this SDMA schemeand the CSMA/CA access scheme adopted in the IEEE802.11 standard, therecan be a situation in which a radio base station cannot spatiallyseparate packets, which are spatially multiplexed and transmitted, froma plurality of radio terminals and cannot receive packets.

When, for example, the radio base station transmits data packets ofdifferent packet lengths spatially multiplexed to the plurality of radioterminals, since response packets are sent back from the respectiveradio terminals at different timings and this results in interference.If the radio base station is assumed to transmit the next data packetafter a lapse of a predetermined time after the reception of theresponse packets, the radio base station transmits the next data packetand receives the response packets simultaneously, and this also resultsin interference (collision).

In order to solve these problems, studies are underway on methods forpreventing the radio base station from transmitting data packets andreceiving response packets simultaneously.

There are disclosed, for example, a method whereby packets aremultiplexed and transmitted from a radio base station to a plurality ofradio terminals (over a downlink) at the same packet transmission timingand its period (e.g., JP-A 2005-39728 (Kokai)) and a method wherebypackets are multiplexed and transmitted from a plurality of radioterminals to a radio base station (over an uplink) at the same packettransmission timing and its period (e.g., JP-A 2003-52079 (Kokai)).

Furthermore, there is also disclosed a method of identifying a group ofa plurality of radio terminals capable of multiplexing and transmittingpackets to a radio base station under an SDMA scheme (e.g., JP-A2007-208522 (Kokai)).

The IEEE802.11 standard defines a mechanism whereby when a radiocommunication apparatus (including a radio base station, radio terminal)receives a packet directed to another radio communication apparatus, aNAV (Network Allocate Vector) is set so as to prevent transmission ofpackets for a certain period of time.

Therefore, when constructing a wireless LAN system compliant with theIEEE802.11 standard and adopting an SDMA scheme, it is necessary to takeinto consideration that the radio communication apparatus which hasreceived a packet directed to the other radio communication apparatusprevents transmission of the packet for a certain period of time.However, e.g., JP-A 2003-52079 (Kokai), JP-A 2005-39728 (Kokai) and JP-A2007-208522 (Kokai) do not take into consideration, when constructing awireless LAN system adopting an SDMA scheme, that the radiocommunication apparatus which has received a packet directed to theother radio communication apparatus prevents transmission of the packetfor a certain period of time.

Furthermore, the IEEE802.11e standard defines a period (TXOP(Transmission Opportunity) period) during which packets aretransmitted/received continuously without relinquishing a transmissionaccess right and allows a plurality of packets to be transmittedcontinuously within the TXOP period.

That is, radio communication apparatuses compliant with the IEEE802.11estandard can transmit a packet within a TXOP period, receive a responsepacket to the packet and then transmit packets continuously withoutacquiring any transmission access right again after a lapse of apredetermined time (e.g., time interval between packets defined by theIEEE802.11 standard MAC protocol specification (SIFS: Short Inter FrameSpace)).

However, the wireless LAN system compliant with the IEEE802.11 standardand adopting an SDMA scheme does not consider the method whereby aplurality of radio terminals spatially multiplex and transmit packets tothe radio base station and continuously transmit packets within the TXOPperiod.

DISCLOSURE OF THE INVENTION

According to an aspect of the present invention, there is provided witha radio system having a first radio terminal, a second radio terminaland a base station, in which the first radio terminal and the secondradio terminal transmit radio signals to the radio base station under aspace multiplexing scheme,

the first radio terminal comprising a first transmitting unit configuredto transmit a request signal to the radio base station for requesting anaccess right to transmit radio signals to the radio base station,

the second radio terminal comprising a prevention unit configured toprevent transmission of radio signals for predetermined duration whenreceiving radio signals other than those directed to the second radioterminal,

the radio base station comprising:

a receiving unit configured to receive the request signal from the firstradio terminal;

a storage configured to store an identifier of a radio terminal whichcan transmit radio signals to the radio base station under the spacemultiplexing scheme in association with the first radio terminal; and

a second transmitting unit configured to transmit an assignment signalfor assigning an access right to transmit radio signals to the radiobase station to not only the first radio terminal which is a sender ofthe request signal, but also the second radio terminal when anidentifier of the second radio terminal is stored in association withthe first radio terminal in the storage,

wherein the first radio terminal transmits radio signals according tothe assignment signal,

the second radio terminal cancels the prevention of transmission ofradio signals according to the assignment signal to transmit radiosignals even when transmission of radio signals is prevented accordingto receiving of the request signal directed to the radio base station,and

a period during which the first radio terminal transmits the radiosignals to the radio base station at least partially overlaps a periodduring which the second radio terminal transmits the radio signals tothe radio base station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a radio system according to a first embodiment of thepresent invention;

FIG. 2 is a time chart showing radio signals transmitted/receivedbetween a radio base station and a radio terminal;

FIG. 3 is a time chart showing radio signals transmitted/receivedbetween a radio base station and a radio terminal;

FIG. 4 shows packet configurations of an RTS packet and a CTS packet;

FIG. 5 is a block diagram showing a configuration of a radio terminalaccording to the first embodiment of the present invention;

FIG. 6 is a block diagram showing a radio base station according to thefirst embodiment of the present invention;

FIGS. 7A and 7B are a time chart showing radio signalstransmitted/received between a radio base station and a radio terminal;

FIG. 8 is a block diagram showing a configuration of a radio terminalaccording to a second embodiment of the present invention;

FIG. 9 is a time chart showing radio signals transmitted/receivedbetween a radio base station and a radio terminal;

FIG. 10 is a time chart showing radio signals transmitted/receivedbetween a radio base station and a radio terminal;

FIG. 11 is a time chart showing radio signals transmitted/receivedbetween a radio base station and a radio terminal;

FIG. 12 is a time chart showing radio signals transmitted/receivedbetween a radio base station and a radio terminal;

FIG. 13 is a block diagram showing a configuration of a radio terminalaccording to a third embodiment of the present invention; and

FIG. 14 is a block diagram showing a configuration of a radio basestation according to the third embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be explained.

First Embodiment

FIG. 1 shows a radio system according to a first embodiment of thepresent invention. This radio system is constructed in compliance withthe IEEE802.11 standard.

The radio system according to the first embodiment is provided withradio terminals STA1 to STA6 and a radio base station AP. The radioterminals STA1 to STA6 and the radio base station AP mutuallytransmit/receive radio signals via radio channels. The radio basestation AP behaves as an access point. FIG. 1 shows a situation in whichthe radio base station AP transmits/receives radio signals to/from theradio terminals STA1 to STA3 under a space multiplexing schemesimultaneously and using the same frequency channel.

When the radio terminals STA1 to STA3 simultaneously transmit radiosignals to the radio base station AP under a space multiplexing scheme,FIGS. 2 and 3 are time charts showing the radio signalstransmitted/received between the radio terminals STA1 to STA3 and theradio base station AP.

First, suppose the radio terminal STA1 has acquired a transmission rightafter a back-off period. The radio terminal STA1 transmits an accessright request signal (e.g., RTS (Request To Send) packet) requesting aright to transmit radio signals to the radio base station AP(hereinafter referred to as “access right”) to the radio base stationAP. This access right request signal (RTS packet) is received by notonly the radio base station AP but also by the radio terminals STA2,STA3 and other radio terminals STA4 to STA6.

The access right request signal (RTS packet) describes duration untilthe transmission of an access right assignment signal (e.g., CTS (ClearTo Send) packet), which is a response signal to the access right requestsignal (RTS packet) is completed (hereinafter referred to as a“transmission prevention duration”).

The radio terminals STA2 to STA6, which have received the access rightrequest signal (RTS packet) whose destination is not the own radioterminal, prevent the transmission of radio signals for the transmissionprevention duration (e.g., value of Duration field) (set (NAV (NetworkAllocation Vector)).

In order for the radio terminal STA1 to transmit radio signals to theradio base station AP simultaneously with other radio terminals under aspace multiplexing scheme, the access right request signal describes aperiod during which the radio terminal STA1 and the other radioterminals transmit radio signals under the space multiplexing scheme(hereinafter referred to as an “SDMA period”).

Next, the radio base station AP, which has received the access rightrequest signal in which the SDMA period is described, transmits anaccess right assignment signal (CTS packet) to not only the radioterminal STA1 but also radio terminals to which radio signals can betransmitted under the space multiplexing scheme simultaneously with theradio terminal STA1 (here, suppose radio terminals STA2 and STA3).

The access right assignment signal (CTS packet) describes the period(SDMA period) during which a plurality of radio terminals (here, radioterminals STA1 to STA3) transmit radio signals under the spacemultiplexing scheme. Here, suppose the SDMA period described in the CTSpacket is the same as the SDMA period described in the RTS packet.

Next, after receiving the CTS packet and being assigned an access right,the radio terminal STA1 transmits a data signal to the radio basestation AP so that the transmission of the data signal is completed attiming at which the SDMA period described in the CTS packet ends.

After receiving the CTS packets and being assigned access rights, theradio terminals STA2 and STA3 cancel the transmission prevention of theradio signals (cancel NAV). The radio terminals STA2 and STA3 thentransmit data signals to the radio base station AP so that thetransmission of the data signals is completed at timing at which theSDMA period described in the CTS packets ends as in the case of theradio terminal STA1.

The radio terminals STA4 to STA6 which have received the access rightassignment signals (CTS packets) whose destinations are not the ownradio terminals prevent the transmission of radio signals for atransmission prevention duration (value of Duration field) (set NAV).

The transmission prevention duration (value of Duration field) describedin the access right assignment signal (CTS packet) is duration duringwhich a series of exchanges between the radio terminals STA1 to STA3 andthe radio base station AP is performed (here, duration untiltransmission of an Ack packet is completed).

Next, the radio base station AP receives the data signals from the radioterminals STA1 to STA3 for the SDMA period and then sends back responsesignals indicating the situation in which the data signals have beenreceived (e.g., Ack packets) to the radio terminals STA1 to STA3.

As described above, the radio terminals STA1 to STA3 simultaneouslytransmit radio signals to the radio base station AP under the spacemultiplexing scheme.

In the above explanations, the access right request signal is assumed tobe an RTS packet defined by the IEEE802.11 standard, the access rightassignment signal is assumed to be a CTS packet and the response signalis assumed to be an ACK packet, but the present invention is not limitedto this. For example, the response signal may also be a Block ACK packetor the like.

Furthermore, when the radio base station AP transmits CTS packets to theradio terminals STA1 to STA3, the addresses of the radio terminals STA1to STA3 may be specified as multicast addresses or an address of a groupof radio terminals that perform transmission to the radio base stationAP under the space multiplexing scheme (hereinafter referred to as an“SDMA group”) may be specified.

FIG. 4 shows packet configurations of an RTS packet and a CTS packet.

Compared to the IEEE802.11 standard, the RTS packet and CTS packet aredifferent in that they include a field for reporting an SDMA period.

The address of a radio terminal, which is a sender of an RTS packet, andthe addresses of radio terminals to which radio signals can betransmitted simultaneously with the radio terminal under the spacemultiplexing scheme are set as the destination addresses of the CTSpackets.

FIG. 5 is a block diagram showing a configuration of the radio terminalSTA1 according to the first embodiment. The configurations of the radioterminals STA2 and STA3 are also the same.

The radio terminal according to the first embodiment is provided with anantenna 10, a radio unit 20, a modulation/demodulation unit 30 and a MACprocessing unit 40. The modulation/demodulation unit 30 is provided witha modulation unit 31, a demodulation unit 32 and a carrier sensing unit33. The MAC processing unit 40 is provided with a transmission unit 41,a reception unit 42, a MAC control unit 43, a memory 44 and an SDMAcontrol unit 45.

The memory 44 stores an SDMA period and various types of controlinformation.

The MAC control unit 43 may also incorporate the SDMA control unit 45.The MAC control unit 43 may also incorporate the SDMA control unit 45and memory 44.

First, an overview of the operation of the radio terminal STA1 whentransmitting radio signals will be explained. The operations of theradio terminals STA2 and STA3 are also the same.

First, data to be transmitted is outputted from a higher layerprocessing unit and inputted to the transmission unit 41 of the MACprocessing unit 40.

Next, the transmission unit 41 performs processing such as adding a MACheader to the inputted transmission data under the control of the MACcontrol unit 43. This transmission data is stored as a transmissionpacket in a transmission buffer (not shown) incorporated in thetransmission unit 41.

Next, the transmission unit 41 outputs transmission packets stored inthe transmission buffer to the modulation unit 31 in order in which theyare stored.

Next, the modulation unit 31 performs processing such as codingprocessing, modulation processing and physical layer related processingsuch as addition of a physical header on the inputted transmissionpacket.

Next, the transmission packet subjected to the physical layer relatedprocessing is subjected to D/A (Digital To Analog) conversion processingand frequency conversion to a frequency band of radio communication inthe radio unit 20 and transmitted through the antenna 10.

In this way, the radio terminal STA1 transmits radio signals.

Next, an overview of the operation of the radio terminal STA1 whenreceiving radio signals will be explained. The operations of the radioterminals STA2 and STA3 are also the same.

First, radio signals received through the antenna 10 is subjected toprocessing of frequency conversion to a baseband and A/D (Analog ToDigital) conversion processing in the radio unit 20 and outputted to thedemodulation unit 32.

Next, the demodulation unit 32 performs physical layer relatedprocessing such as demodulation processing and an analysis of thephysical header on the packet outputted from the radio unit 20.

Furthermore, the demodulation unit 32 reports Received Signal StrengthIndicator (RSSI) of the received radio signal to the carrier sensingunit 33.

Next, the demodulation unit 32 then outputs the received packetsubjected to the physical layer related processing to the MAC processingunit 40.

Next, the reception unit 42 of the MAC processing unit 40 performs MAClayer related processing such as a MAC header analysis.

Next, the reception unit 42 of the MAC processing unit 40 outputs thereceived packet to a higher layer processing unit when the destinationof the received packet is the own radio terminal.

In this way, the radio terminal STA1 receives the radio signal.

Next, an overview of the operation of the radio terminal STA1 when radiosignals are transmitted simultaneously with other radio terminals underthe space multiplexing scheme (hereinafter referred to as “SDMAtransmission”) will be explained. The operations of the radio terminalsSTA2 and STA3 are also the same.

First, the processing until a transmission packet is stored in thetransmission buffer is the same as the normal transmission processing inthe radio terminal STA1.

Next, when transmitting an RTS packet to the radio base station, thetransmission unit 41 inquires of the SDMA control unit 45 about whetheror not to perform SDMA transmission.

Next, the SDMA control unit 45 decides whether or not to permit SDMAtransmission in response to the inquiry from the transmission unit 41.The decision method of the SDMA control unit 45 will be described later.

Next, when the SDMA transmission is permitted, the transmission unit 41describes the SDMA period in the RTS packet and performs transmissionprocessing.

On the other hand, when the SDMA transmission is not permitted, thetransmission unit 41 does not describe the SDMA period in the RTS packetor describes “0” in the SDMA period and performs transmissionprocessing.

Next, the radio terminal STA1 transmits radio signals according to a CTSpacket sent back from the radio base station AP. Here, the operation ofthe radio terminal STA2 will be shown when the radio terminal STA1 hastransmitted an RTS packet and the radio terminal STA2 has received a CTSpacket. The operation of the radio terminal STA3 is also the same.

Hereinafter, the CTS packet reception processing of the radio terminalSTA2 will be explained.

First, the CTS packet is received through the antenna 10 and subjectedto reception processing in the radio unit 20 and demodulation unit 32.

Next, the CTS packet directed to the own radio terminal STA2 is inputtedto the reception unit 42 of the MAC processing unit 40.

Next, upon recognizing that the CTS packet directed to the own radioterminal STA2 has been inputted, the reception unit 42 of the MACprocessing unit 40 reports cancellation of NAV to the MAC control unit43.

Furthermore, upon recognizing that the CTS packet directed to the ownradio terminal STA2 has been inputted, the reception unit 42 of the MACprocessing unit 40 reports the SDMA period described in the CTS packetto the SDMA control unit 45.

Next, upon receiving the report of the SDMA period from the receptionunit 42, the SDMA control unit 45 decides whether or not SDMAtransmission is possible from the information (e.g., size) oftransmission packets stored in the transmission buffer, currenttransmission rate when transmitting a transmission packet and the lengthof the SDMA period.

That is, the SDMA control unit 45 calculates the time required totransmit the transmission packet from the information (e.g., size) ofthe transmission packets stored in the transmission buffer and thecurrent transmission rate when transmitting a transmission packet. Whenthe calculation result is equal to or less than the length of the SDMAperiod, the SDMA control unit 45 decides that the SDMA transmission ispossible.

Next, when the SDMA transmission is decided to be possible, the SDMAcontrol unit 45 calculates timing to start transmission of a data signal(hereinafter referred to as “transmission start timing”) so that the endtime of the SDMA period coincides with the time at which transmission ofthe packet is completed.

Next, the SDMA control unit 45 reports a transmission instruction andtransmission start timing to the transmission unit 41.

In this way, even when no RTS packet is transmitted, the radio terminalSTA2 (radio terminal STA3) which has received a CTS packet operates andthereby allows the radio terminal STA1 which is the sender of the RTSpacket and the radio terminal STA2 (radio terminal STA3) tosimultaneously transmit radio signals to the radio base station underthe space multiplexing scheme.

FIG. 6 is a block diagram showing a configuration of the radio basestation according to the first embodiment.

The radio base station according to the first embodiment is providedwith an antenna 110, a radio unit 120, a modulation/demodulation unit130 and a MAC processing unit 140. The modulation/demodulation unit 30is provided with a modulation unit 131, a demodulation unit 32 and acarrier sensing unit 133. The MAC processing unit 140 is provided with atransmission unit 141, a reception unit 142, a MAC control unit 143, amemory 144 and an SDMA control unit 145.

The memory 144 stores an SDMA period, various types of controlinformation and information of a group (hereinafter referred to as an“SDMA group”) of radio terminals to which radio signals can besimultaneously transmitted under the space multiplexing scheme. Theinformation of the group includes identifiers of the radio terminalsbelonging to the same group, for example. The number of the radioterminals belonging to the group may be either one or plural number.

The MAC control unit 143 may incorporate the SDMA control unit 145. TheMAC control unit 143 may also incorporate the SDMA control unit 145 andmemory 144.

Since the operation of the radio base station AP whentransmitting/receiving radio signals is the same as the operationexplained in the radio terminal STA1, explanations thereof will beomitted. Hereinafter, the operation of the radio base station AP whensimultaneously receiving radio signals from a plurality of radioterminals under the space multiplexing scheme will be explained.

First, the operation when the radio base station AP receives an RTSpacket in which an SDMA period is described from the radio terminal STA1will be explained.

First, an RTS packet is received through the antenna 110 and subjectedto reception processing in the radio unit 120 and demodulation unit 132.

Next, the RTS packet in which an SDMA period is described is inputted tothe reception unit 142 of the MAC processing unit 140.

Next, the reception unit 142 of the MAC processing unit 140 decideswhether or not the SDMA period is described in the RTS packet.

When the SDMA period is not described in the RTS packet, the radio basestation AP performs processing of sending back a CTS packet to the radioterminal which is the sender of the RTS packet.

On the other hand, when the SDMA period is described in the RTS packet,the reception unit 142 of the MAC processing unit 140 sends a reportindicating the presence of an SDMA transmission request to the SDMAcontrol unit 145.

Triggered by the report indicating the presence of the SDMA transmissionrequest, the SDMA control unit 145 reads information on which radioterminals (e.g., radio terminals STA2 and STA3) can be simultaneouslytransmitted with the sender of the RTS packet (e.g., radio terminalSTA1) under the space multiplexing scheme from the memory 144. Theinformation includes, for example, identifiers of the radio terminalswhich can simultaneously transmit with the sender of the RTS packetunder the space multiplexing scheme.

Next, the SDMA control unit 145 reports to the transmission unit 141that the radio terminal STA1, which is the sender of the RTS packet, andthe radio terminals STA2 and STA3 to which radio signals can besimultaneously transmitted under the space multiplexing scheme will bethe destination addresses of the CTS packets.

Furthermore, the SDMA control unit 145 reports the length of the SDMAperiod described in the RTS packet to the transmission unit 141.

Next, the transmission unit 141 sets the destination addresses(addresses of the radio terminals STA1 to STA3) of the CTS packetsaccording to the report from the SDMA control unit 145 and sets the SDMAperiods of the CTS packets. The transmission unit 141 transmits the CTSpackets in which the destination addresses and the SDMA periods are set.

In this way, upon receiving an RTS packet in which an SDMA period isdescribed from the radio terminal STA1, the radio base station AP sendsback CTS packets.

The group of radio terminals to which radio signals can besimultaneously transmitted under the space multiplexing scheme (e.g.,radio terminal STA1 which is the sender of the RTS packet and radioterminals to which radio signals can be simultaneously transmitted underthe space multiplexing scheme) can be grasped, for example, by the radiobase station AP inquiring of the respective radio terminals about theirrelationships with other radio terminals such as a radio channelenvironment. The method of grasping the group of radio terminals towhich radio signals can be simultaneously transmitted under the spacemultiplexing scheme is not limited to the method described above and,for example, the technique disclosed in JP-A 2007-208522 (Kokai) andtechniques disclosed in other documents can be used.

Second, the operation when the radio base station AP receives RTSpackets in which an SDMA period is described from the plurality of radioterminals STA1 to STA3 will be explained.

When a radio base station that does not transmit/receive radio signalsunder the space multiplexing scheme (e.g., conventional radio basestation compliant with the IEEE802.11 standard) simultaneously receivesa plurality of radio signals (e.g., RTS packets), interference(collision) occurs and this prevents the radio base station fromperforming reception.

However, a radio base station AP that can transmit/receive radio signalsunder the space multiplexing scheme can simultaneously receive aplurality of radio signals (e.g., RTS packets). For example, a radioterminal that belongs to a group capable of spatial multiplexing (e.g.,radio terminals STA1 to STA3) can simultaneously transmit RTS packets tothe radio base station AP.

Hereinafter, the operation of the radio base station AP uponsimultaneously receiving RTS packets from a plurality of radio terminalsSTA1 to STA3 will be explained.

First, the RTS packets transmitted from the plurality of radio terminalsSTA1 to STA3 are received through the antenna 110 and subjected toreception processing in the radio unit 120 and demodulation unit 132.

Next, the reception unit 142 of the MAC processing unit 140 receives theRTS packets transmitted from the plurality of radio terminals STA1 toSTA3 as input.

Next, the reception unit 142 of the MAC processing unit 140 decides oneRTS packet out of the plurality of inputted RTS packets according towhich the reception unit 142 performs processing of sending back a CTSpacket.

The method for the reception unit 142 of the MAC processing unit 140 toselect an RTS packet according to which the reception unit 142 performsthe processing of sending back a CTS packet will be explained below.Suppose, the radio base station AP has simultaneously received RTSpackets from the plurality of radio terminals STA1 to STA3.

A first method is a method of determining a radio terminal which has alow frequency with which an access right is acquired (frequency withwhich a CTS packet is received or frequency with which a CTS packet isreceived in response to an RTS packet transmitted by itself) out of theplurality of radio terminals which are the senders of a plurality of RTSpackets and selecting the RTS packet transmitted from the determinedradio terminal.

A second method is a method of determining an SDMA group which has a lowfrequency with which an access right is acquired (frequency with which aCTS packet is received or frequency with which a CTS packet is receivedin response to an RTS packet transmitted by itself) out of the pluralityof radio terminals which are the senders of a plurality of RTS packetsand selecting the RTS packet transmitted from the determined SDMA group.

A third method is a method of determining a radio terminal which has along period that has elapsed after acquiring the last access right(period that has elapsed after receiving the last CTS packet or periodthat has elapsed after receiving a CTS packet in response to an RTSpacket transmitted by itself) out of a plurality of radio terminalswhich are the senders of a plurality of RTS packets and selecting theRTS packet transmitted from the determined SDMA group.

A fourth method is a method of determining an SDMA group which has along period that has elapsed after acquiring the last access right(period that has elapsed after receiving the last CTS packet or periodthat has elapsed after receiving a CTS packet in response to an RTSpacket transmitted by itself) out of a plurality of SDMA groups whichare the senders of a plurality of RTS packets and selecting the RTSpacket transmitted from the radio terminal belonging to the determinedSDMA group.

A fifth method is a method of selecting, when RTS packets aresimultaneously received from a plurality of radio terminals STA1 to STA3belonging to the same SDMA group, an RTS packet in which a longest SDMAperiod is described out of the SDMA periods described in the pluralityof RTS packets.

In the first embodiment, the method for the reception unit 142 of theMAC processing unit 140 to select an RTS packet according to which theMAC processing unit 140 performs processing of sending back a CTS packetis not limited to the above described selection method.

In this way, according to the radio system of the first embodiment, theradio base station transmits, when receiving an access right requestsignal from a certain radio terminal, an access right assignment signal,which is a response thereto, to not only the radio terminal which is thesender of the access right assignment signal but also radio terminalswhich can transmit radio signals to the own radio base stationsimultaneously with the radio terminal under the space multiplexingscheme, and it is thereby possible for a plurality of radio terminals tosimultaneously transmit radio signals to the radio base station underthe space multiplexing scheme.

According to the radio system of the first embodiment, even when anaccess right request signal transmitted by a certain radio terminal to aradio base station is received by another radio terminal and thetransmission of the radio signal is prevented by the other radioterminal, if the other radio terminal which prevents the transmission ofthe radio signal receives the access right assignment signal, aplurality of radio terminals can simultaneously transmit radio signalsto the radio base station under the space multiplexing scheme bycanceling the prevention of the transmission of the radio signal.

As the protocol when a plurality of radio terminals simultaneouslytransmit radio signals to a radio base station under the spacemultiplexing scheme, it is possible to provide a protocol inconsideration of NAV defined in IEEE802.11.

Furthermore, even when a radio base station simultaneously receivesaccess right request signals from a plurality of radio terminals underthe space multiplexing scheme, it is possible to provide a protocol inconsideration of NAV defined in IEEE802.11 as the protocol when theplurality of radio terminals simultaneously transmit radio signals tothe radio base station under the space multiplexing scheme.

Modification Example 1

The above described first embodiment assumes that an SDMA period isdescribed in an RTS packet transmitted by the radio terminal STA1 to theradio base station AP. However, the effect by the radio system accordingto the first embodiment can be achieved even if the SDMA period is notdescribed in the RTS packet transmitted by the radio terminal STA1 tothe radio base station AP and an RTS packet configuration defined in theIEEE802.11 standard is adopted.

The radio terminal STA1 describes the duration until a series ofexchanges between the radio terminal STA1 and radio base station AP iscompleted (e.g., until the time at which the reception of the Ack packetshown in FIG. 3 ends) in the value of the “Duration” field of the RTSpacket.

In the example in FIG. 3, the radio terminal STA1 describes a valueresulting from adding up SIFS, CTS reply time, SIFS, length of SDMAperiod, SIFS and Ack reply time as the value of the Duration field ofthe RTS packet.

Even if the SDMA period is not described in the RTS packet, the radiobase station AP calculates the SDMA period in consideration of the timerequired for transmission of the CTS packet and ACK packet or the like.

In the example of FIG. 3, the radio base station AP can calculate theresult of subtracting SIFS, CTS reply time, SIFS, SIFS and Ack replytime from the value of the Duration field of the RTS packet as the SDMAperiod.

The radio base station AP may also always receive radio signals from apredetermined radio terminal simultaneously with other radio terminalsunder the space multiplexing scheme.

When a radio terminal which is the sender of an RTS packet belongs toany one SDMA group, the radio base station AP may also receive radiosignals from the radio terminal which is the sender of the RTS packetsimultaneously with the other radio terminals under the spacemultiplexing scheme.

Furthermore, the radio terminal STA1 may report whether or not toperform transmission/reception of radio signals under the spacemultiplexing scheme using part of an RTS packet transmitted from theradio terminal STA1 to the radio base station AP.

Whether or not the radio terminal STA1 performs transmission/receptionof radio signals under the space multiplexing scheme may be described ina signal field of a PHY header of an RTS packet or Frame Control of aMAC header.

This radio base station AP and radio terminals STA1 to STA3 can also berealized using, for example, a general-purpose computer as basichardware. That is, the transmission unit 41, 141, reception unit 42,142, MAC control unit 43, 143, and SDMA control unit 45, 145 can berealized by causing a processor mounted on the computer to execute aprogram. In this case, the radio base station AP and radio terminalsSTA1 to STA3 may also be realized by installing the program in thecomputer beforehand or storing the program in a storage medium such as aCD-ROM or distributing the program via a network and installing theprogram in the computer as appropriate. Furthermore, the memory 44, 144and transmission buffer may also be realized using the memory 44, 144and hard disk incorporated in or externally provided for the computer ora storage medium such as a CD-R, CD-RW, DVD-RAM, and DVD-R asappropriate.

Embodiment 2

As the radio system according to the first embodiment, the mechanism fora plurality of radio terminals to transmit radio signals to a radio basestation under the space multiplexing scheme simultaneous has beenexplained.

The radio system according to a second embodiment uses a concept of“TXOP period” defined in the IEEE802.11e standard during which packetsare transmitted/received without relinquishing any access right.

In the radio system according to the second embodiment, the radioterminal describes duration until the TXOP period ends in thetransmission prevention duration (e.g., value of Duration field) of anaccess right request signal (e.g., RTS packet).

FIGS. 7A and 7B are a time chart showing radio signalstransmitted/received between the radio terminals STA1 to STA3 and radiobase station AP when the radio terminals STA1 to STA3 continuouslytransmit radio signals to the radio base station AP under SDMA within aTXOP period. A plurality of SDMA periods are included in the TXOPperiod. TXOP period corresponds to a first period and each SDMA periodcorresponds to a second period respectively, for example.

Here, an RTS packet is described in abbreviation as “R”, CTS packet as“C”, data packet as “DATA” and Ack packet as A.

FIG. 7A is a time chart when an access right request signal (RTS packet)and an access right assignment signal (CTS packet) aretransmitted/received between the radio terminals STA1 to STA3 and radiobase station AP for every SDMA period.

FIG. 7B is a time chart when an access right request signal (RTS packet)and an access right assignment signal (CTS packet) are nottransmitted/received and exchanges of RTS packets and CTS packets areomitted between the radio terminals STA1 to STA3 and radio base stationAP for every SDMA period.

Each SDMA period included in the TXOP period has the same length. Thelength of each SDMA period is determined by the SDMA period inserted inthe CTS packet sent back by the radio base station to the radio terminalSTA1 that has transmitted the first RTS packet.

The configurations of the RTS packet and CTS packet according to thesecond embodiment are similar to those of the RTS packet and CTS packetaccording to the first embodiment shown in FIG. 4.

The configuration of the radio base station according to the secondembodiment is similar to the configuration of the radio base stationaccording to the first embodiment shown in FIG. 6.

FIG. 8 is a block diagram showing a configuration of a radio terminalSTA1 according to the second embodiment. The configurations of radioterminals STA2 and STA3 are also the same. Explanations of the sameparts of the radio terminal STA1 according to the second embodiment asthose of the radio terminal STA2 according to the first embodiment willbe omitted.

The radio terminal STA1 according to the second embodiment is providedwith an antenna 10, a radio unit 20, a modulation/demodulation unit 30and a MAC processing unit 40. The modulation/demodulation unit 30 isprovided with a modulation unit 31, a demodulation unit 32 and a carriersensing unit 33. The MAC processing unit 40 is provided with atransmission unit 41, a reception unit 42, a MAC control unit 43, amemory 44 and an SDMA control unit 45.

The memory 44 stores a TXOP period and SDMA period described in a CTSpacket sent back from the radio base station AP and various types ofcontrol information.

The SDMA control unit 45 decides whether or not transmission of furtherdata packets is possible during the TXOP period according to the TXOPperiod and SDMA period stored in the memory 44.

That is, triggered by the reception of a response signal (Ack frame) inresponse to a data packet which has already been transmitted, the SDMAcontrol unit 45 decides whether or not a new SDMA period can be reservedwithin the remaining TXOP period in consideration of the TXOP period,SDMA period, time required to receive a response signal and SIFS or thelike.

When deciding that transmission of further data packet is possible, theSDMA control unit 45 instructs the transmission unit 41 to transmit thenext data packet.

Instead of deciding whether or not transmission of the next data packetis possible every time a response signal (Ack frame) is received, theSDMA control unit 45 may calculate the number of SDMA periods that canbe reserved within the TXOP period beforehand and transmit the samenumber of data packets as the number of times of the calculation result.That is, the SDMA control unit 45 may calculate the number of datapackets that can be transmitted within the TXOP period beforehand andthereby decide whether or not transmission of the next packet ispossible.

In this way, according to the radio system of the second embodiment,even when a plurality of radio terminals simultaneously transmit radiosignals to the radio base station under the space multiplexing scheme,it is possible to provide a mechanism of continuously transmitting radiosignals within a period (TXOP period) during which radio signals can becontinuously transmitted without relinquishing the access right andimprove transmission efficiency and reception efficiency of the radiosignal.

Embodiment 3

When a plurality of radio terminals continuously transmit radio signalsto a radio base station within a TXOP period, the radio system of athird embodiment shows a mechanism of flexibly changing the length ofSDMA period and the plurality of radio terminals (SDMA group) thattransmit radio signals to the radio base station under SDMA.

Example 1

FIGS. 9 and 10 are time charts showing radio signalstransmitted/received between the radio terminals STA1 to STA3 and radiobase station AP in the radio system according to the third embodiment ofthe present invention.

First, in order to acquire an access right to the radio base station AP,the radio terminal STA1 transmits an access right request signal (RTSpacket) to the radio base station AP. The radio base station APtransmits an access right assignment signal (CTS packet) to the radioterminals STA1 to STA3. In this way, the TXOP period and SDMA period aredetermined as in the case of the radio system of the second embodiment.

The radio terminals STA2 and STA3, which have not transmitted any RTSpacket to the radio base station AP, yet have received a CTS packet,transmit a data packet according to the SDMA period described in the CTSpacket.

The radio terminals STA2 and STA3 attempt to transmit data packetsaccording to the request of the radio terminal STA1 (RTS packet) so thatthe transmission of data packets is completed within the set SDMAperiod.

However, depending on the length of the SDMA period set according to therequest of the radio terminal STA1, there can be a situation in whicheven a radio terminal (e.g., radio terminal STA3) that can performtransmission together with the radio terminal STA1 under the spacemultiplexing scheme cannot transmit a data packet. One such example is asituation in which the SDMA period is so short that even when any one ofdata packets stored in the transmission buffer of the radio terminalSTA3 is transmitted, the transmission is not completed within the SDMAperiod.

Although the radio terminal STA3 has already received the CTS packetfrom the radio base station AP and data packets are stored in thetransmission buffer, the radio terminal STA3 cannot transmit any datapacket because the length of the SDMA period is not appropriate.

The radio terminal STA3 transmits a change request signal (controlsignal) for requesting a change of the length of the SDMA period insteadof the data packet to the radio base station AP during the SDMA period.The radio terminal STA3 transmits the change request signal so that thetransmission of the change request signal is completed when the SDMAperiod ends.

FIGS. 9 and 10 have shown an example where a “QoS Null” packet is usedas a change request signal. However, the change request signal onlyneeds to have a field that allows the SDMA period desired by the sourceradio terminal to be described and may be a signal defined in otherIEEE802.11 standards or a newly defined signal.

Next, upon receiving the change request signal, the radio base stationAP decides whether to change the SDMA period or change the radioterminal that performs SDMA transmission (SDMA group) from the currentlyset SDMA period and the SDMA period described in the change requestsignal.

Next, upon deciding that the SDMA period or SDMA group can be changed,the radio base station AP sends back a response signal (Ack packet)describing “SDMA enabled to change.”

On the other hand, upon deciding that the SDMA period or SDMA groupcannot be changed, the radio base station AP sends back a responsesignal (Ack packet) describing “SDMA disabled to change” or sends back anormal response signal (Ack packet).

Next, the radio terminals STA1 to STA3 receive the above describedresponse signal (Ack packet).

When “SDMA disabled to change” is described in the above describedresponse signal (Ack packet) or when the above described response signal(Ack packet) is a normal Ack packet, the radio terminals STA1 to STA3transmit data packets according to the currently set SDMA period.

On the other hand, when “SDMA enabled to change” is described in theabove described response signal (Ack packet), the radio terminal STA3which has transmitted the change request signal receives the abovedescribed response signal and then transmits an RTS packet describing adesired SDMA period to the radio base station AP after a lapse of SIFS.

Next, the radio base station AP transmits a CTS packet describing anSDMA period desired by the radio terminal STA3 to the radio terminalsSTA1 to STA3.

When changing the SDMA period, the radio base station AP substitutes thevalue of the changed SDMA period into the SDMA period field of the CTSpacket shown in FIG. 4.

Furthermore, when changing the SDMA group, the radio base station APtransmits the above described CTS packet to a plurality of radioterminals belonging to a new SDMA group. The radio terminal which hasreceived the CTS packet from the radio base station AP then restarts thetransmission of data packets.

In this way, the SDMA period and SDMA group are changed within the TXOPperiod.

By so doing, the radio base station AP needs only to decide whether ornot it is possible to change the SDMA period or SDMA group during theSIFS period after QoS Null is received from the radio terminal STA3until a response signal (Ack packet) is transmitted, and can therebysuppress the processing speed required for the radio base station APcompared, for example, to a case where the changed SDMA period isreported.

The above explanation has assumed that the radio terminal STA3, whichhas transmitted QoS Null, transmits an RTS packet. However, the SDMAperiod requested by the radio terminal STA3 has already been reported tothe radio base station AP through the change request signal. Therefore,the radio terminal STA1 may transmit an RTS packet to the radio basestation AP instead of the radio terminal STA3 transmitting an RTS packetto the radio base station AP. That is, when desiring the suspension ofthe change of the SDMA period, suppose the radio terminal STA1 transmitsan RTS packet to request the holding of the currently set SDMA period.

By so doing, it is possible to prevent an SDMA period which isundesirable for the radio terminal STA1 which has acquired the firstaccess right to the radio base station AP from being set.

Example 2

FIGS. 11 and 12 are time charts showing radio signalstransmitted/received between the radio terminals STA1 to STA3 and theradio base station AP in the radio system according to the thirdembodiment of the present invention.

In Example 1 shown in FIGS. 9 and 10, the radio terminal STA3 transmitsa change request signal and then transmits an RTS packet in which adesired SDMA period is described.

However, after the radio base station AP receives the change requestsignal, “SDMA enabled to change” may be described and a response signal(Ack packet) in which the changed SDMA period is described may betransmitted. By so doing, transmission/reception of an RTS packet andCTS packet between the radio terminal STA3 and the radio base station APcan be omitted.

First, in Example 2, the radio base station AP receives a change requestsignal (QoS Null) from the radio terminal STA3 as in the case of Example1 and decides whether or not to change the SDMA period.

Upon deciding to change the SDMA period, the radio base station APtransmits a response signal (Ack packet) in which “SDMA enabled tochange” and changed SDMA period are described to the radio terminalsSTA1 to STA3.

Next, upon receiving a response signal (Ack packet), the radio terminalsSTA1 to STA3 decide “SDMA enabled/disabled to change” from the responsesignal.

Upon deciding “SDMA disabled to change,” the radio terminals STA1 toSTA3 continue to transmit data packets after SIFS according to thecurrently set SDMA period.

On the other hand, upon deciding “SDMA enabled to change,” the radioterminals STA1 to STA3 continue to transmit data packets after SIFSaccording to the changed SDMA period described in the response signal.

Although the SDMA group cannot be changed, this makes it possible toomit, when changing the SDMA period within the TXOP period, exchanges ofRTS packets and CTS packets and is therefore efficient.

Two examples (Example 1 and Example 2) have been explained above, butthe radio base station AP may also change the SDMA period using any oneof the method explained in Example 1 and the method explained in Example2.

The radio base station AP may also use different methods, such asadopting the method explained in Example 1 when changing the SDMA groupand adopting the method explained in Example 2 when only changing theSDMA period.

Furthermore, the explanations in Example 1 and Example 2 assume thesituation in which since the SDMA period set by the radio terminal STA1is so short that the radio terminal STA3 cannot transmit data packets.

However, on the contrary, there can also be a situation in which theSDMA period set by the radio terminal STA1 is so long that the radioterminal STA3 cannot transmit data packets efficiently.

Furthermore, the radio terminal STA3 may also use an RTS packet withouttransmitting it to report that although the radio terminal STA3 hasreceived a CTS packet, no data packet to be transmitted to the radiobase station AP is stored in the transmission buffer.

For example, the radio terminal STA3 may report that no data packet tobe transmitted to the radio base station AP is stored in thetransmission buffer to the radio base station AP by sending an RTSpacket in which the SDMA period is set to “0.” In this case, uponreceiving the RTS packet in which the SDMA period is set to “0,” theradio base station AP can decide to change the SDMA group.

Example 3

FIG. 13 is a block diagram showing a configuration of a radio terminalSTA1 according to a third embodiment. The configurations of radioterminals STA2 and STA3 are also the same. Hereinafter, the differencesfrom the radio terminal according to the second embodiment (operation ofthe SDMA control unit 45) will be mainly explained.

The SDMA control unit 45 of the radio terminal STA1 decides whether ornot to desire a change of an SDMA period. For example, the SDMA controlunit 45 decides whether or not to desire a change of the SDMA perioddepending on whether transmission of a data packet is possible withinthe current SDMA period.

When it is decided that the change of the SDMA period is desired, theSDMA control unit 45 instructs the transmission unit 41 to transmit achange request signal (QoS Null). The SDMA control unit 45 may alsoindicate the value of a desired SDMA period as well when instructing thetransmission of the change request signal.

Upon receiving a response signal (Ack packet) in which “SDMA enabled tochange” is described and also the SDMA period is described from theradio base station AP, the SDMA control unit 45 of the radio terminalSTA1 instructs the transmission unit 41 to transmit a data packetaccording to the SDMA period described in the response signal.

Upon receiving the response signal (Ack packet) in which “SDMA enabledto change” from the radio base station AP, the SDMA control unit 45 ofthe radio terminal STA1 decides whether or not to desire a change of theSDMA period and instructs, when the change is desired, the transmissionunit 41 to transmit a signal in which the SDMA period is described.

FIG. 14 is a block diagram showing a configuration of the radio basestation AP according to the third embodiment. Hereinafter, thedifferences from the radio base station according to the secondembodiment will be mainly explained.

Suppose the radio base station AP has received a change request signal(RTS packet) from a radio terminal. The SDMA control unit 145 decideswhether to change the SDMA period or change the SDMA group from thecurrent SDMA period and SDMA period information described in the RTSpacket.

Upon deciding that the SDMA period or SDMA group is to be changed, theSDMA control unit 145 instructs the transmission unit 141 to transmit aresponse signal (Ack packet) in which “SDMA enabled to change” isdescribed.

When the SDMA period is changed, the SDMA control unit 145 may alsoreport the changed SDMA period to the transmission unit 141 togetherwith an instruction to transmit a response signal in which “SDMA enabledto change” is described.

According to the radio terminals STA1 to STA3 and the radio base stationaccording to the third embodiment, when any one of the plurality ofradio terminals that simultaneously transmit radio signals to the radiobase station AP under the space multiplexing scheme cannot transmitradio signals at the current transmission rate due to influences ofsituations of radio channels or the like, the radio terminal can performretransmission at a reduced transmission rate.

The IEEE802.11 standard defines a technique whereby when transmission ofradio signals fails, retransmission is performed at a reducedtransmission rate. However, when retransmission is performed at areduced transmission rate, the packet transmission time becomes longer.

Here, if the packet transmission time for retransmission at the reducedtransmission rate is within the SDMA period, there is no problem.However, there can be a situation in which the packet transmission timefor retransmission at the reduced transmission rate exceeds the SDMAperiod.

In this case, since each of the plurality of radio terminals thatsimultaneously transmit radio signals under the space multiplexingscheme is allowed to change the SDMA period, it is possible to supportretransmission at the reduced transmission rate.

That is, by setting a longer SDMA period than the packet transmissiontime when retransmission is performed at the reduced transmission rate,data packets can be retransmitted at the reduced transmission rate.

When a plurality of radio terminals continuously transmit radio signalsto the radio base station under the space multiplexing scheme, byproviding a mechanism that makes it possible to flexibly change theperiod (TXOP period) of transmitting radio signals under SDMA and agroup (SDMA group) of radio terminals that transmit radio signals underSDMA even during an SDMA period during which radio signals aretransmitted under SDMA, it is possible to increase the possibility oftransmission/reception of radio signal between a plurality of radioterminals and the radio base station, and thereby improve throughput ofthe overall system.

Embodiment 4

The third embodiment has explained the case where the radio base stationreceives a signal for requesting a change of the SDMA period (RTSpacket) from one radio terminal.

The fourth embodiment will explain a case where a radio base stationreceives change request signals from a plurality of radio terminals.

Upon receiving change request signals from a plurality of radioterminals STA1 to STA3, a radio base station AP determines whether ornot to send back a response signal according to the change requestsignal transmitted from any one of the radio terminals.

A first method is a method of selecting a change request signal from aradio terminal which has acquired TXOP and sending back a responsesignal thereto.

A second method is a method of selecting, after the radio base stationAP fails to receive a packet transmitted from a certain radio terminal(the radio base station AP has received some signal level but failed todecode the packet), a change request signal from the radio terminal andsending back a response signal thereto.

A third method is a method of selecting, when change request signals arereceived from a plurality of radio terminals STA1 to STA3 belonging tothe same SDMA group, a change request signal of a longest SDMA perioddescribed out of the plurality of change request signals and sendingback a response signal thereto.

A fourth method is a method of selecting a change request signal from aradio terminal having a maximum number of retransmissions and sendingback a response signal thereto.

The radio base station can use any one of the above described fourmethods or combine the above described four methods.

In this way, according to the radio system of the fourth embodiment,even when the radio base station simultaneously receives SDMA periodchange requests from a plurality of radio terminals, it is possible toflexibly change an SDMA period and SDMA group.

Therefore, it is possible to increase the possibility oftransmission/reception of radio signals between a plurality of radioterminals and a radio base station and improve throughput of the overallsystem.

The present invention is not limited to the exact embodiments describedabove and can be embodied with its components modified in animplementation phase without departing from the scope of the invention.Also, arbitrary combinations of the components disclosed in theabove-described embodiments can form various inventions. For example,some of the all components shown in the embodiments may be omitted.Furthermore, components from different embodiments may be combined asappropriate.

1. One of a plurality of radio terminals, comprising: a receiving unitconfigured to receive a request signal from another radio terminal ofthe plurality of radio terminals, the request signal comprises a requestfor an access right to transmit radio signals to a radio base station,the request signal describes a first period and a second period, thesecond period being shorter than the first period; a prevention unitconfigured to prevent transmission of radio signals upon receiving therequest signal; and a transmitting unit configured to transmit radiosignals to the radio base station during the first period according tothe assignment signal received from the radio base station, and the oneradio terminal completes the transmission of the radio signals withinthe second period.
 2. The terminal according to claim 1, wherein thesecond radio terminal ends transmission of the radio signal when thesecond period described in the assignment signal ends.
 3. A radio basestation, comprising: a receiving unit configured to receive an accessright request from a first radio terminal, the access right requestidentifies a first period and a second period; and a transmitting unitconfigured to transmit an assignment signal to the first radio terminaland the second radio terminal, the first period and the second periodbeing described in the assignment signal, wherein the receiving unitreceives radio signals transmitted from the first radio terminal and thesecond radio during the first period, and wherein transmission of theradio signals by the first radio terminal and the second radio terminalcompletes within the second period.
 4. The station according to claim 3,wherein the second radio terminal cancels the prevention of transmissionof radio signals and transmits radio signals according to the assignmentsignal even when transmission of radio signals is prevented according toreceiving of the request signal directed to the radio base station, andthe first radio terminal transmits radio signals according to theassignment signal.
 5. The station according to claim 3, wherein therequest signal describes a duration during which transmission of radiosignals to another radio terminal other than the first radio terminal isprevented, a transmission period of radio signals by the first radioterminal is calculated based on the duration described in the requestsignal, and the assignment signal describes the calculated transmissionperiod of the radio signals by the first radio terminal as a periodduring which radio signals can be transmitted to the radio base stationunder the space multiplexing scheme.
 6. The station according to claim3, wherein when the receiving unit receives the request signals from aplurality of radio terminals, the receiving unit assumes that therequest signal has been received from a radio terminal having a lowfrequency with which the access right has been given out of theplurality of radio terminals or a radio terminal having a longer timethat has elapsed after being given the access right out of the pluralityof radio terminals.
 7. The station according to claim 3, wherein thereceiving unit receives the request signals from a plurality of radioterminals which can transmit radio signals to the radio base stationunder the space multiplexing scheme, the request signals received fromthe plurality of radio terminals describe transmission periods of theradio signals requested by the plurality of radio terminalsrespectively, and the assignment signal describes a longest period outof transmission periods of radio signals requested by the plurality ofradio terminals as a period during which radio signals can betransmitted to the radio base station under the space multiplexingscheme.
 8. A radio base station, comprising: a receiving unit configuredto receive, from a first radio terminal, a request signal for requestingan access right to transmit radio signals to the radio base station; anda transmitting unit configured to transmit an assignment signal forassigning an access right to the first radio terminal and a second radioterminal, wherein an identifier of the second radio terminal isassociated with the first radio terminal, wherein the request signaldescribes a first period and a second period, the second period isshorter than the first period, the assignment signal describes the firstperiod and the second period, the receiving unit receives radio signalsfrom the first radio terminal and the second radio terminal during thefirst period; and the first radio terminal and the second radio terminalcomplete transmission of the radio signals within the second period. 9.The station according to claim 8, further comprising a decision unitconfigured to decide, when the receiving unit receives a change requestsignal for requesting a change of the length of the second period fromthe first radio terminal or the second radio terminal during the firstperiod, whether or not to change the length of the second period basedon a current length of the second period and the length of the secondperiod requested by the first radio terminal or the second radioterminal, which is the sender of the change request signal, and whetheror not to change a group of radio terminals that receive radio signalsunder the space multiplexing scheme from a group including the first andsecond radio terminals to another group.
 10. The station according toclaim 9, wherein when the decision unit decides to change the length ofthe second period, the transmitting unit transmits a signal forreporting the changed second period to the first radio terminal and thesecond radio terminal, and when the decision unit decides to change thegroup of radio terminals to the another group, the transmitting unittransmits a signal for reporting the changed second period to theanother group.
 11. The station according to claim 9, wherein when thereceiving unit receives the change request signals from a plurality ofradio terminals, the decision unit performs decision processing based onthe length of the second period requested by a radio terminal alreadyassigned the access right out of the plurality of radio terminals andthe current length of the second period.
 12. The station according toclaim 9, wherein when the receiving unit receives the change requestsignals from a plurality of radio terminals, the decision unit performsdecision processing based on the length of the second period requestedby one radio terminal determined out of the plurality of radio terminalswherein the one radio terminal is determined according to receptionsituations of the radio signals from the plurality of radio terminalsunder the space multiplexing scheme.
 13. The station according to claim9, wherein when the receiving unit receives the change request signalsfrom a plurality of radio terminals, the decision unit performs thedecision processing based on a longest period of the second periodsrequested by the plurality of radio terminals or the length of thesecond period requested by a radio terminal having a maximum number ofretransmissions out of the plurality of radio terminals.